Preparation Of B₄C Two-dimensional Nanosheet Composites And Study Of Its Electrocatalytic Nitrogen Reduction Properties

NH₃is an important precursor for industrial synthetic fertilizers and many other nitrogen-containing chemicals.In addition,NH₃also shows the prospect of being a hydrogen energy carrier in the upcoming era of renewable energy.The source of NH₃is mostly from the conversion of N₂.Although there is a large amount of N₂in the atmosphere,the triple bond in N₂is very stable and difficult to open,so N₂is difficult to be used.The Haber-Bosch reaction is an important technology that can convert N₂into NH₃.However,the high temperature and high pressure environment makes the consumed energy account for about 2%of the total global energy consumption,resulting in a large amount of carbon dioxide emissions and environmental pollution.In view of the shortage of fossil fuels and global climate change,there is an urgent need to develop a sustainable,energy-saving and environmentally friendly synthetic ammonia process.Electrochemical catalytic technology provides a new synthesis idea for the conversion of nitrogen to ammonia.In this catalytic process,the equipment is relatively simple,and the catalytic conditions are low temperature and low pressure,and the raw materials used are abundant nitrogen and water.Based on the unique energy source of the electric energy in the electrocatalytic process,so wind energy,solar energy and thermal energy can all provide clean and sustainable energy.In this paper,B₄C nanosheets were selected as the base material and then combined with Nb₂O₅nanoparticles and Bi nanoparticles respectively.The synthesized composites were used for the electrocatalytic nitrogen fixation test.The morphology,crystal structure and valence structure of the prepared B₄C nanosheet-based composite catalyst were characterized by scanning electron microscopy,X-ray diffractometry,X-ray photoelectron spectroscopy and transmission electron microscopy.Uv-visible spectrophotometers are used to measure the concentration of ammonium ions in the electrolyte,and the performance parameters such as the rate of ammonia production and the Faraday efficiency of electrocatalytic nitrogen fixation are calculated according to the formula to evaluate the performance of ammonia.Main research contents are concluded as mentioned below:（1）B₄C nanosheets were prepared by ultrasonicating B₄C ethanol solution with an ultrasonic cleaner.The results of electrocatalytic nitrogen fixation experiments show that the bulk B₄C material has the best performance at-0.6 V vs.RHE,the ammonia yield is11.2 ug/h/mgcat.,and the Faraday efficiency is 3%.The electrocatalytic nitrogen fixation performance of B₄C nanosheets has the best performance at-0.6 V vs.RHE.The ammonia yield is 19.5 ug/h/mgcat.and the Faraday efficiency is 13%.The performance improvement of B₄C nanosheets is attributed to the increase of its specific surface area and the increase of active sites.（2）The B₄C nanosheets were directly added during the synthesis of Nb₂O₅nanoparticles by the traditional sol-gel method,so that the Nb₂O₅nanoparticles were directly grown on the B₄C nanosheets to synthesize the composite.The Nb₂O₅nanoparticles/B₄C nanosheet composites have the best performance at-0.2 V vs.RHE by electrocatalytic nitrogen fixation.The ammonia yield is 7.4 ug/h/mg_cat.and the Faraday efficiency is 36.5%.（3）The B₄C nanosheet and Bi Cl₃were co-dispersed into a dimethyl sulfoxide（DMSO）solution,and heated at 120℃for 8 h in a heating mantle to obtain a Bi nanoparticle/B₄C nanosheet composite.Bi nanoparticles can effectively inhibit hydrogen production at low voltage and act as an active point to catalyze nitrogen.According to the electrocatalytic nitrogen fixation test,the catalyst has the best ammonia yield of 17.1ug/h/mg_cat.at-0.4 V vs.RHE,and the best Faraday efficiency of 53.5%at-0.3 V vs.RHE.